1. Field of the Invention
The present invention relates to a joint driving apparatus for a robot.
2. Description of the Related Art
For example, International Publication No. WO2007/032400A1 has proposed an apparatus for driving joint portions of a robot. As shown in FIG. 6, the joint driving apparatus 10 is fixed to a base 12 constituting a part of a robot (not illustrated) and supports and drives a movable member 14 constituting another part of the robot so as to rotate freely.
The joint driving apparatus 10 is equipped with a reduction mechanism 18 for reducing the rotation of a motor 16. A driving power of the motor 16 is transmitted in order of an input gear (not illustrated) mounted on a motor shaft 20, an external gear 22, one eccentric body shaft 24 (illustrated), and an eccentric body shaft gear 25 provided on the eccentric body shaft 24, and reaches a center gear 23. The center gear 23 is meshed with eccentric body shaft gears 25 (not illustrated) provided respectively on other two eccentric body shafts 24 (not illustrated), thereby rotating three eccentric body shafts 24 (only above one shaft is illustrated).
Eccentric bodies 26A, 26B are integrally formed on the eccentric body shaft 24. The eccentric bodies 26A, 26B rotate in an eccentric manner, then external gears 28A, 28B eccentrically rotate while being internally meshed with an internal gear 30.
The joint driving apparatus 10 is constituted in such a manner that a rotational component of an eccentric rotation of these external gears 28A, 28B is output through first and second carriers 32A, 32B (output members) and transmitted to the movable member 14 via a bolt 34.
A through hole 36 is formed axially at a center of the joint driving apparatus 10. A control cable (wire harness) 40 for controlling the posture of the robot penetrates the through hole 36. A seal mechanism made with a hollow cylinder 41 and O-rings 43, 45 is provided for sealing inside and outside the reduction mechanism 18. An oil seal 44A is arranged between the first carrier 32A and a casing 31 for sealing inside and outside of the reduction mechanism 18, and an oil seal 44B for the same purpose is arranged between the second carrier 32B and the casing 31.
In the joint driving apparatus 10 of the robot, the oil seals 44A, 44B for sealing inside and outside of the reduction mechanism 18 are arranged between the first carrier 32A and the casing 31, and between the second carrier 32B and the casing 31 respectively. This is because that in a case where the first carrier 32A and the second carrier 32B, which are output members, are arranged at the axial side of the external gear, and where the oil seals 44A, 44B are arranged between the first or second carrier 32A or 32B and the first member of the robot respectively, it is the most natural and easiest way to arrange the oil seals 44A, 44B at the outermost peripheral portion of the first and second carriers 32A, 32B.
However, in the above constitution, the oil seals 44A, 44B are extremely large in diameter to result in a larger seal area, thus easily lowering the sealing stability against leakage. Further, although the output members (the first and second carriers 32A, 32B) are basically low in peripheral speed, a fast speed is found at the outer peripheral portion where the diameter is large, thereby easily causing wear due to sliding motions. Thus, a problem is posed that the service life of the oil seals 44A, 44B is shortened, thereby, making it difficult to maintain stable sealing characteristics for a long time.